Crystals of HCMV protease diffract anisotropically and never beyond 3.5 _ in the weak directions with laboratory X-ray sources. Native and 6 heavy atom derivative data sets were collected at CHESS beam line F1 (>95% complete to 2.8 _ resolution in 4 cases) on one of the crystal forms of human cytomegalovirus protease. Although significant problems with non-isomorphism were encountered, useful phase information was obtained from one derivative. Low resolution phases (4 _) and a partial model are available from a related crystal form. Molecular replacement solutions of the CHESS data based on the partial model are being evaluated.